This application is based on Japanese Patent Application No. 2001-212973 filed in Japan, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
This invention relates to a material for liquid-crystal alignment films, a liquid-crystal display element and a process for its production, and a liquid-crystal unit.
2. Description of the Related Art
As liquid-crystal display elements used in liquid-crystal units and so forth, those of a TFT (thin-film transistor) switching system are widely used. In recent years, improvements in their display performance are being achieved, and liquid-crystal alignment films used therein have become required to satisfy various characteristics simultaneously.
Liquid crystal alignment film characteristics required in this TFT switching system are known to include, as disclosed in, e.g., Japanese Patent Application Laid-open No. 10-197875, pre-tilt angles of liquid-crystal molecules, voltage holding ratio, and residual DC (direct current) voltage of direct-current voltage, as well as adherence to substrates with transparent electrodes, and printability free of any non-uniformity or pinholes. Meanwhile, in order to meet requirements for higher precision in future liquid-crystal display elements and for changes in display modes, liquid-crystal alignment films are required to have good step (difference in level)-covering properties against any differeneces in surface levels of TFT substrates.
Liquid-crystal alignment films are commonly obtained by coating a solution of a soluble polyimide or polyimide precursor on a substrate, followed by baking to effect curing to form a polyimide film, which is then subjected to rubbing to make alignment. In order to improve the step-covering properties, it may be contemplated to make the soluble polyimide or polyimide precursor solution have a higher solid-matter concentration to form a thicker film so that the differences in levels can be covered. However, in usual cases, solvent-soluble polyimide or polyimide precursors are so low soluble that there has been a limit to the improving of step-covering properties by making the solid-matter concentration higher.
Accordingly, an object of the present invention is to provide a material for a liquid-crystal alignment film which satisfies fundamental prescribed properties such as voltage holding ratio, pre-tilt angles, residual DC voltage, adherence to substrates, and printability, and also has superior step-covering properties, and to provide a liquid-crystal display element making use of this material, a process for producing the device, and a liquid-crystal display unit having the device.
To achieve the above object, the present invention provides a material for a liquid-crystal alignment film which comprises:
(i) a first polyamide having a first repeating unit represented by the following Formula (1) and having a weight-average molecular weight of from 5,000 to 200,000; 
wherein R1 is a tetravalent organic group represented by any of the following structural formulas (2), R2 is a divalent organic group, and R3 is an alkyl group having 3 to 10 carbon atoms or a fluoroalkyl group having 3 to 10 carbon atoms; 
(ii) a second polyamide having a second repeating unit represented by the following Formula (3) and a third repeating unit represented by the following Formula (5), and having a weight-average molecular weight of from 5,000 to 200,000. The number of the third repeating unit with respect to the number of total repeating units in one molecule of the second polyamide may preferably be in a proportion of from 1 mole % to 50 mole %. 
wherein R4 is a tetravalent organic group represented by the following Formula (4), R5 is a divalent organic group, and R6 is an alkyl group having 3 to 10 carbon atoms or a fluoroalkyl group having 3 to 10 carbon atoms. 
wherein X is a saturated or unsaturated hydrocarbon group. 
wherein R7 is a straight-chain alkyl group having 6 to 20 carbon atoms or a straight-chain fluoroalkyl group having 6 to 20 carbon atoms.
The present invention also provides a liquid-crystal display element comprising:
two substrates each having an electrode (i.e. at least one electrode), wherein the electrode of one substrate and the electrode of another substrate are disposed opposingly to each other;
a liquid crystal held between the substrates; and
a liquid-crystal alignment film disposed between the liquid crystal and the electrode;
the liquid-crystal alignment film each comprising a cured product of the material for a liquid-crystal alignment film according to the present invention;
and also provides a liquid-crystal display unit having the device.
The present invention still also provides a process for producing a liquid-crystal display element; the process comprising:
a resin film formation step for forming the above material for a liquid-crystal alignment film according to the present invention into a film on a substrate having an electrodes to obtain a resin film; and
a rubbing step for rubbing the resin film to make the resin film into a liquid-crystal alignment film.